1. Field of the Invention
This invention relates generally to an apparatus and method for making downhole measurements during the drilling of a wellbore. More particularly, it relates to an apparatus and method for making downhole measurements at or near the drill bit during directional drilling of a wellbore.
2. Background of the Related Art
The drilling of oil and gas prospecting wells, also know as “boreholes,” typically involves the use of a drilling assembly—particularly a directional drilling assembly—for penetrating one or more subsurface formations of interest. Such drilling assemblies, also known as “drill strings,” typically include lower sections known as “bottom-hole assemblies” or BHAs. A BHA may consist (generally from the bottom up in a vertical borehole section) of a drill bit, bit sub, stabilizers, drill collars, directional drilling equipment, measurement-while-drilling (MWD) tools, logging-while-drilling (LWD) tools, drill pipe, and other specialized devices. The MWD tools and LWD tools acquire data that is representative of various drilling, drill bit, and formation parameters. Acquired drilling parameters may include the direction and inclination (D&I), and toolface of the BHA; acquired drill bit parameters may include measurements such as weight on bit (WOB), torque on bit and drive shaft speed; and acquired formation parameters may include resistivity (or conductivity), natural radiation, density (natural gamma ray or neutron emission), pore pressure, and other parameters that characterize the formation.
MWD tools are often equipped with an integrated telemetry system and are placed in communication with LWD tools (or integrated therewith), so that the telemetry system of the MWD tools may be employed for transmitting real time or near-real time signals representing the acquired drilling, drill bit, and formation parameters to the surface for processing and interpretation. Examples of MWD tools and associated telemetry systems are described in U.S. Pat. No. 5,375,098 by Malone et al., U.S. Pat. No. 5,517,464 by Lerner et al., and U.S. Pat. No. 6,219,301 to Moriarty, each of which is assigned to the assignee of the present invention.
When a conventional MWD tool is used in combination with a mud motor (i.e., in a drill string that employs a mud motor), the MWD tool is located above the mud motor at a substantial distance from the drill bit. Thus, it is not unusual for an MWD tool to be positioned as much as 40 feet above the drill bit, considering the length of a non-magnetic spacer collar and other components that are typically connected in the drill string between the MWD tool and the motor. Such substantial distances between the sensors in the MWD tool (and possibly a LWD tool) and the drill bit mean that the sensors' measurements of the downhole conditions (e.g., drilling, drill bit, and/or formation parameters) are made a substantial time after the drill bit has passed the formation location from which the measurements are taken. Therefore, if there is a need to adjust the borehole trajectory based on information from the MWD/LWD sensors, the drill bit will have already traveled some considerable distance before the need to adjust its direction is apparent. Adjustment of the borehole trajectory under these circumstances can be a difficult and costly task. Accordingly, there is a desire in many drilling applications, especially when drilling directional wells, to make the downhole measurements as close to the drill bit as possible.
The intentional directional control of a borehole, commonly known as “directional drilling,” may be based upon three-dimensional targets in space (e.g., from seismic images), upon the results of downhole geological logging measurements (e.g., by geosteering), or upon other information with the usual objective of targeting or keeping a directional borehole within a valuable formation (i.e., within a so-called “pay zone”). Directional drilling is generally achieved by pointing or pushing the drill bit in the intended borehole direction. The most common way of pointing the bit is through the use of a bend near the bit in a downhole steerable mud motor assembly. The bend is commonly introduced by a bent housing in the mud motor assembly, which is selectively rotated to point-the-bit in a direction different from the axis of the wellbore. Then, when mud is pumped through the mud motor while the drill string rotation is stopped, the bit is rotated about its axis to drill in the direction the bit has been pointed. The bent housing may employ a fixed or adjustable bend. One example of an adjustable bent housing is described in U.S. Pat. No. 5,117,927, assigned to the assignee of the present invention.
Another way of pointing the drill bit, or, alternatively, of pushing the bit, for directional drilling purposes is achieved through a rotary steerable system (RSS), also known as a rotary steerable tool (RST). A RSS allows directional drilling to proceed while the drill string is rotating, usually with higher rates of penetration and ultimately smoother boreholes than are achievable with mud motors. A RSS (embodiments of which are described in detail below) may include D&I sensors, system control electronics, power generation equipment, and communication links, in addition to its steering components. Since many of these elements are provided in a MWD tool, there will be some redundancy in a drill string employing a RSS with a MWD tool. Thus, with the advancement of RSS and MWD/LWD technology, it has become advantageous to consider their integration into a single assembly in order to move measurement sensors closer to the bit and reduce costs downhole through the elimination of duplicate elements and functionalities.
Additionally, since drilling time equates to increased cost, it is generally preferred to conduct drilling at the highest rate possible, i.e., to achieve the highest rate of penetration (ROP) through the subsurface earth. This often dictates that the drill string be rotated at speeds that approach the limits of the rotary table, thereby increasing the wear on the drill string and the casing. For this reason (as well as others, such as enhanced performance), the practice of combining a RSS (which directs the bit while the drill string is rotated) with a mud motor (which directs the bit while the drill string is not rotating) has recently been proposed. The above-mentioned desire to make downhole measurements (e.g., MWD/LWD) as close to the drill bit as possible also applies to such combinations of a mud motor with a RSS.
In conventional drilling operations, a borehole is drilled to a selected depth with a drill string having numerous interconnected joints of heavyweight pipe called drill pipe (as well as a BHA as described herein), and then the borehole is lined with a larger-diameter pipe called casing. Casing typically consists of larger-diameter pipe joints connected end-to-end, similar to the way the drill pipe is connected. To accomplish the setting of casing in the borehole, the drill string including the BHA are removed from the borehole in a process called “tripping.” Once the drill string is removed, the casing is lowered into the borehole and cemented in place. The casing protects the borehole from collapse and isolates the subsurface formations from each other. After the casing is set in place, drilling may continue.
Conventional drilling typically includes a series of drilling, tripping, casing, and cementing sequences that are repeated again and again as the borehole penetrates the subsurface earth. This process is very time consuming and costly. Additionally, other problems are often encountered when tripping the drill string. For example, the drill string (or a portion thereof) may get stuck in the borehole while it is being removed. These problems require additional time and expense to resolve. As a result, the practice of casing drilling (also called liner drilling in some instances), wherein casing is employed as the drill pipe, has recently been commercialized. In casing drilling, a BHA including a drill bit are connected to the lower end of a casing string, and the borehole is drilled using the casing string to transmit mud, as well as axial and rotational forces, to the drill bit. Upon completion of drilling, the casing string may then be cemented in place to form the casing for the borehole. Casing drilling thus enables the borehole to be simultaneously drilled and cased.
Casing drilling is adaptable to the employment of measuring and directional drilling tools/systems as described herein. Examples of casing drilling strings that employ mud motors and MWD tools are described in U.S. Pat. No. 5,197,553 by Leturno, U.S. Pat. No. 6,196,336 by Fincher et al., and U.S. Patent Application Publication No. 2004/0026126 by Angman. Examples of casing drilling strings that employ RSS and MWD tools are described in U.S. Pat. No. 6,419,033 by Hahn et al., and U.S. Pat. No. 6,705,413 by Tessari. However, the desires and shortcomings identified above (among others) concerning conventional drilling strings are evident in these casing drilling strings.